1. Field of the Invention
The present invention relates to an apparatus for recording a video signal through frequency modulation on a recording medium such as a magnetic tape.
2. Description of the Prior Art
In video tape recorders, particularly color video tape recorders, it is customary for better signal-to-noise ratio of reproduced video signals to preemphasize a luminance signal of a composite color video signal, i.e., emphasize high frequency components of the luminance signal, then frequency-modulate the preemphasized luminance signal, and record the frequency-modulated luminance signal on a magnetic tape. When the recorded luminance signal is reproduced, it is frequency-demodulated and then deemphasized into a luminance signal with the original frequency characteristics.
When a luminance signal is preemphasized, an abrupt transient from a low level portion to a high level portion of the luminance signal turns into a spike (white spike) which is responsible for a picture image reversal or a poor signal-to-noise ratio. In order to avoid such a drawback, it has been proposed to clip spikes caused by high and low level portions of a preemphasized luminance signal before the preemphasized luminance signal is frequency-modulated as disclosed in Japanese Laid-Open Patent Publication No. 59(1984)-208989.
Some conventional video tape recorders employ clipping circuits for removing spikes. The earlier clipping circuits have a constant clipping level for shearing off spikes of a preemphasized luminance signal irrespective of whether the signal is to be recorded on an evaporated tape or a metal tape.
The evaporated and metal tapes will briefly be described below.
The evaporated tape comprises a tape base on which there is deposited a thin film of magnetic metal (e.g., Co-Ni alloy) with no binder, by vacuum evaporation. The tape base and deposited film of magnetic metal of the obliquely evaporated tape are thinner than those of coated tapes, the tape base and deposited film of magnetic metal being typically 10 .mu.m thick. Obliquely evaporated tape is obliquely magnetized, while metal powder coated tape is horizontally magnetized, which gives bigger demagnetization in recording short wave lengths. Therefore, the evaporated tape is suitable for high-density recording and is high in performance for recording in short wavelengths. The evaporated tape has high coercivity, a good squareness ratio, and improved self-demagnetization loss and thickness loss due to the oblique evaporated thin-film configuration. The residual magnetism of the evaporated tape is several times greater than that of the cobalt-ferric-oxide tape.
The metal tape comprises a tape base coated with fine particles (whose diameter is 1 .mu.m or less) or a 100% crystal of a ferromagnetic metal such as iron, cobalt, nickel, or the like. The metal tape has a magnetic energy (i.e., the area defined by the hysteresis loop) which is four times that of the cobalt-ferric-oxide tape, and exhibits high performance for short-wavelength recording.
The evaporated and metal tapes have the following characteristics:
______________________________________ Evaporated tape Metal tape ______________________________________ Saturated flux density (gauss) 7,500 3,000 Residual flux density (gauss) 6,000 2,300 Coercive force (Oe) 800 1,300 Magnetic layer thickness (.mu.m) 0.1 2.5 Tape thickness (.mu.m) 10 17 ______________________________________
FIGS. 1A and 1B show signal-to-noise ratios of frequency-modulated luminance signals which are reproduced from evaporated and metal tapes, respectively, while the tapes are running for long periods of time, with the preemphasized luminance signals being clipped with a constant clipping level regardless of the types of the tapes in a conventional video tape recorder, i.e., a high-band 8-mm (Hi 8) video tape recorder.
In each of FIGS. 1A and 1B, the horizontal axis represents time (hour) and the vertical axis the signal-to-noise ratio of a frequency-modulated luminance signal which is reproduced from a magnetic tape. The vertical axes of FIGS. 1A and 1B are of identical graduations indicated in the same units, although they are not shown.
FIG. 1A shows the signal-to-noise ratio of signal components in 7 MHz and 10 MHz of a frequency-modulated luminance signal which is reproduced from an evaporated tape, whereas FIG. 1B shows the signal-to-noise ratio of signal components in 7 MHz, 9 MHz, and 10 MHz of a frequency-modulated luminance signal which is reproduced from a metal tape.
A comparison between the characteristic curves shown in FIGS. 1A and 1B indicates that the signal-to-noise ratios (FIG. 1B) of the higher-frequency signal components of the frequency-modulated luminance signal reproduced from the metal tape drop to a greater extent with time than the signal-to-noise ratios (FIG. 1A) of the same higher-frequency signal components of the frequency-modulated luminance signal reproduced from the evaporated tape. This means that the signal-to-noise ratios are reduced with time for different reasons with respect to the evaporated and metal tapes.
More specifically, when an evaporated tape is held in sliding contact with the contact surface of a rotary magnetic head, the contact surface of the rotary magnetic head is worn by the evaporated tape, and therefore, the distance between the contact surface of the rotary magnetic head and the magnetic layer of the evaporated tape varies with time. Consequently, the signal-to-noise ratio of the frequency-modulated luminance signal which is reproduced from the evaporated tape by the rotary magnetic head is lowered with time.
When a metal tape is held in sliding contact with the contact surface of a rotary magnetic head, a thin film of fine ferromagnetic particles from the metal tape is deposited on the contact surface of the rotary magnetic head. For this reason, the signal-to-noise ratio of the frequency-modulated luminance signal which is reproduced from the metal tape by the rotary magnetic head is also lowered with time.
There has been proposed a data reproducing system in which audio data recorded on a magnetic tape by biphase pulse code modulation are reproduced by a magnetic head, and information data are demodulated from the reproduced audio data, the data reproducing system having an equalizer with different equalizing characteristics that are selectable depending on the type of the magnetic tape used, as disclosed in Japanese Laid-Open Patent Publication No. 63(1988)-96775.
The clipping circuits of the conventional color video tape recorders have a constant clipping level regardless of widely different carrier-to-noise ratios of the electromagnetic transducer with respect to frequency signal components of frequency-modulated luminance signals reproduced from different types of magnetic tapes. Thus, picture image reversal margins where white spikes are clipped cannot be optimized for all different types of magnetic tapes.